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Xylan oligosaccharides and cellobiohydrolase I (TrCel7A) interaction and effect on activity

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8 pages
The well-studied cellulase mixture secreted by Trichoderma reesei (anamorph to Hypocrea jecorina ) contains two cellobiohydolases (CBHs), cellobiohydrolase I ( Tr Cel7A) and cellobiohydrolase II ( Tr CeI6A), that are core enzymes for the solubilisation of cellulose. This has attracted significant research interest because of the role of the CBHs in the conversion of biomass to fermentable sugars. However, the CHBs are notoriously slow and susceptible to inhibition, which presents a challenge for the commercial utilisation of biomass. The xylans and xylan fragments that are also present in the biomass have been suggested repeatedly as one cause of the reduced activity of CHBs. Yet, the extent and mechanisms of this inhibition remain poorly elucidated. Therefore, we studied xylan oligosaccharides (XOSs) of variable lengths with respect to their binding and inhibition of both Tr Cel7A and an enzyme variant without the cellulose-binding domain (CBM). Results We studied the binding of XOSs to Tr Cel7A by isothermal titration calorimetry. We found that XOSs bind to Tr Cel7A and that the affinity increases commensurate with XOS length. The CBM, on the other hand, did not affect the affinity significantly, which suggests that XOSs may bind to the active site. Activity assays of Tr Cel7A clearly demonstrated the negative effect of the presence of XOSs on the turnover number. Conclusions On the basis of these binding data and a comparison of XOS inhibition of the activity of the two enzyme variants towards, respectively, soluble and insoluble substrates, we propose a competitive mechanism for XOS inhibition of Tr Cel7A with phosphoric swollen cellulose as a substrate.
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Baumannet al.Biotechnology for Biofuels2011,4:45 http://www.biotechnologyforbiofuels.com/content/4/1/45
R E S E A R C H
Open Access
Xylan oligosaccharides and cellobiohydrolase (TrCel7A) interaction and effect on activity 1,2 2 1* Martin J Baumann , Kim Borch and Peter Westh
I
Abstract Background:The wellstudied cellulase mixture secreted byTrichoderma reesei(anamorph toHypocrea jecorina) contains two cellobiohydolases (CBHs), cellobiohydrolase I (TrCel7A) and cellobiohydrolase II (TrCeI6A), that are core enzymes for the solubilisation of cellulose. This has attracted significant research interest because of the role of the CBHs in the conversion of biomass to fermentable sugars. However, the CHBs are notoriously slow and susceptible to inhibition, which presents a challenge for the commercial utilisation of biomass. The xylans and xylan fragments that are also present in the biomass have been suggested repeatedly as one cause of the reduced activity of CHBs. Yet, the extent and mechanisms of this inhibition remain poorly elucidated. Therefore, we studied xylan oligosaccharides (XOSs) of variable lengths with respect to their binding and inhibition of bothTrCel7A and an enzyme variant without the cellulosebinding domain (CBM). Results:We studied the binding of XOSs toTrCel7A by isothermal titration calorimetry. We found that XOSs bind toTrCel7A and that the affinity increases commensurate with XOS length. The CBM, on the other hand, did not affect the affinity significantly, which suggests that XOSs may bind to the active site. Activity assays ofTrCel7A clearly demonstrated the negative effect of the presence of XOSs on the turnover number. Conclusions:On the basis of these binding data and a comparison of XOS inhibition of the activity of the two enzyme variants towards, respectively, soluble and insoluble substrates, we propose a competitive mechanism for XOS inhibition ofTrCel7A with phosphoric swollen cellulose as a substrate. Keywords:cellobiohydrolase 1,TrCel7A, xylan, xylan oligosaccharide, binding, inhibition, biomass degradation, iso thermal titration calorimetry
Background Biomass is the most abundant raw material in the bio sphere, and enzymatic biomass conversion of sugars and subsequent fermentation to transportation fuels has attracted enormous scientific and industrial interest [1]. In typical biomass raw materials such as corn stover or wheat straw, cellulose is not present in pure form but is in close contact with hemicelluloses such as xylan, man nan and xyloglucan and encrusted with lignin [2]. Many thermochemical pretreatments have been developed to disperse and dissolve the recalcitrant complex to make it more easily accessible for enzymatic hydrolysis [3]. Several phenomenological studies have documented a negative influence of xylans and xylan oligosaccharides
* Correspondence: pwesth@ruc.dk 1 Research Unit for Biomaterials, Roskilde University, NSM, Universitetsvej 1, DK4000, Roskilde, Denmark Full list of author information is available at the end of the article
(XOSs) on the performance of industrial cellulase mix tures [47]. However, which enzymes of the commercial cellulases are affected remains unresolved. Furthermore, target sites and mechanisms of these effects remain to be investigated. Such information will clearly be needed when considering remedies for xylan inhibition. The termcellulasemost often refers to secreted cel lulolytic enzymes of fungal origin. For instance, the cel lulases secreted byTrichoderma reesei(anamorph to Hypocrea jecorina) contain several endoglucanases and two cellobiohydrolases (CBHs) which processexoacting, cellobiosereleasing enzymes. The CBHs exhibit quite low reaction rates and nonMichaelisMenten kinetics, which include an initial burst followed by a biphasic slowdown [8,9]. Classical inhibition studies which result in an inhibition constantKiare not applicable to cello biohydrolases, because the concentration of the insolu ble substrate cellulose is very difficult to measure and,
© 2011 Baumann et al; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.